The present invention relates to the field of optically inspecting objects such as printed circuit boards or the like.
In performing optical inspection of P.C. boards, integrated circuits, photomasks or other similar objects, it is often necessary to form high contrast images of the surface patterns of such objects. Under high magnification, "flat" plated conductors, for example, will have portions which are not really flat, and the conductor side edges will not be absolutely at right angles with respect to the "flat" top surfaces of the plated conductors. These edge portions often resemble irregularly sloping surfaces which are to be visually observed. Since the metallic conductors reflect light at least somewhat specularly, and have the aforesaid surface irregularities, these surfaces may be described as non-planar or curved surfaces, having irregular specular topography, although the surfaces are often "flat" in a general sense. In order to obtain images of uniform intensity of a surface with facetted or curved topography, the surface is illuminated with an isotropic light source, i.e., the surface is illuminated uniformly at all angles which may be accomplished by a light integrating cavity for providing a large number of reflections therein.
However, it is observed that certain portions of the object thus illuminated will still appear dark, which is undesirable since information regarding the object's configuration, for example, is thereby lost. Such dark spots will be seen by placing a magnifier over an illuminated P.C. board having metallic conductors thereon. This interesting phenomenon may also be observed by photographing an illuminated mirror "head on" to produce a dark spot at the optical axis. An observer looking at a mirror, however, will not see the spot due to the darkness of the iris and pupil of the eye. In FIG. 1 of this specification, a microscope is employed to view a portion of a P.C. board which is illuminated by an off-axis light source. A simplified schematic representation (in cross section) of the plated conductor 1 is illuminated and light ray 2 will be redirected by the right-hand portion of the conductor and will be visible to the observer employing the microscope to view the P.C. board "head on." However, the flat horizontal portion 3 of the conductor causes light ray 4 to rebound as indicated, so that the observer sees a dark spot. Sloping portion 6 of conductor 1 will also not be seen by the observer since the surface is blocked from the light source by the conductor. If the aforesaid light integrating cavity is positioned about the P.C. board, remote portion 6 would then be illuminated and thus be made visible to the observer. However, since virtually all of the reflected light which emanates from the cavity walls would strike the flat portion 3 at an angle with respect to optical axis 5, the dark spot will still remain as such reflected light would not enter the microscope. Thus, the provision of a light diffusing cavity surrounding the P.C. board having specular patterns thereon of an irregular topography would still produce dark spots. If the microscope used to view the printed circuit board is positioned off-axis, such as in a position occupied by the lamp in FIG. 1 of this specification, the remote slope 6 would not be viewed by the observer through the microscope. See FIG. 2A of U.S. Pat. No. 3,398,634 with respect to this point. It is thus a principal object of the present invention to provide an optical illumination apparatus for viewing all irregular portions of specular objects and yet eliminate the aforesaid dark spot problem produced by portions perpendicular to the optical viewing axis.
In U.S. Pat. No. 4,144,556, an object is placed within an oval shaped shell having a light diffusing inner surface for reducing glare. In U.S. Pat. No. 4,236,781 a light diffusion plate is positioned between a light source and an object to be viewed by an off-axis camera, and in U.S. Pat. No. 3,737,226 a pair of pyramidal shaped diffuser members are employed to cause light to strike the subject being viewed at numerous angles. In U.S. Pat. No. 4,220,982 light passes through a cylindrical diffuser before striking a set of parabolic reflectors. This arrangement also includes a semi-reflecting mirror to block most direct light from striking the object being viewed. None of these references are directed to the solution of the above-stated problem which is the primary object of my invention.